Keck Cosmic Web Imager Observations of He II Emission in I Zw 18

Abstract

With a metallicity of 12 + Log(O/H) $\approx$ 7.1-7.2, I Zw 18 is a canonical low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including I Zw 18, have been found to host strong, narrow-lined, nebular He II ($\lambda$4686) emission with enhanced intensities compared to H$\beta$ (e.g., He II($\lambda$4686)/H$\beta$ > 1%). We present new observations of I Zw 18 using the Keck Cosmic Web Imager. These observations reveal two nebular He II emission regions (or He III regions) northwest and southeast of the He III region in the galaxy's main body investigated in previous studies. All regions exhibit He II($\lambda 4686$)/Hbeta greater than 2%. The two newly resolved He III regions lie along an axis that intercepts the position of I Zw 18's ultraluminous X-ray (ULX) source. We explore whether the ULX could power the two He III regions via shock activity and/or beamed X-ray emission. We find no evidence of shocks from the gas kinematics. If the ULX powers the two regions, the X-ray emission would need to be beamed. Another potential explanation is that a class of early-type nitrogen-rich Wolf-Rayet stars with low winds could power the two He III regions, in which case the alignment with the ULX would be coincidental.

9 pages, 4 figures, accepted for publication in ApJL

References in corpus (9)

• The MAPPINGS III Library of Fast Radiative Shock Models
• I Zw 18 revisited with HST/ACS and Cepheids: New Distance and Age
• X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies
• LZIFU: an emission-line fitting toolkit for integral field spectroscopy data
• The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources
• Pulsing and Non-Pulsing ULXs: the Iceberg Emerges
• Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18
• On the contribution of the X-ray source to the extended nebular HeII emission in IZw18
• Blue galaxies: modelling nebular He II emission in high redshift galaxies

Cited by in corpus (10)

• The COS Legacy Archive Spectroscopy SurveY (CLASSY) Treasury Atlas
• CLASSY IV: Exploring UV diagnostics of the interstellar medium in local high-$z$ analogs at the dawn of the JWST era
• Can nebular HeII emission be explained by ultra-luminous X-ray sources?
• EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-Poor Galaxies: Uncovering the Origins of Strong HeII and the Impact on Cosmic Reionization
• Elevated Hot Gas and High-Mass X-ray Binary Emission in Low Metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe
• Ionization of HeII in star-forming galaxies by X-rays from cluster winds and superbubbles
• The ionizing and heating power of ultraluminous X-ray sources under the geometrical beaming model
• Unraveling the Kinematics of IZw18: A Detailed Study of Ionized Gas with MEGARA/GTC
• New clues on the extended HeII ionization in IZw18 from GTC/MEGARA and JWST/MIRI
• Declining metallicity and extended HeII in the outflow of an epoch of reionization analogue galaxy